Preparation of block copolymers

ABSTRACT

Block copolymers and the processes of preparing them are disclosed wherein alpha-olefins and/or mixtures of alpha-olefins are alternately polymerized in the presence of a catalyst comprising a titanium halide and an aluminum alkyl compound.

This application is a division of application Ser. No. 709,194 filedJan. 16, 1958, now U.S. Pat. No. 3,970,719 issued July 20, 1976.

This invention relates to the preparation of block copolymers. In oneaspect, it relates to a process for preparing novel block copolymers ofaliphatic 1-olefins.

It has recently been discovered that ethylene and other olefins can bepolymerized to high molecular weight solid polymers by utilizing acatalyst system comprising an organo aluminum compound and a halide of agroup IV-A metal. It has also been found that copolymers of ethylene andpropylene can be prepared by a catalytic polymerization in which thecatalyst employed comprises an organo aluminum compound and eithertitanium dichloride or titanium trichloride. In accordance with thisinvention, a novel method is provided for preparing block copolymers ofaliphatic 1-olefins.

It is an object of this invention to provide a process for preparingnovel block copolymers of aliphatic 1-olefins.

Another object of the invention is to provide a catalyst system whichcan be used in the preparation of block copolymers of aliphatic1-olefins.

Other objects and advantages of the invention will become apparent uponconsideration of the accompanying disclosure.

The instant invention resides in a novel catalyst and method forpreparing block copolymers. The polymers prepared by the instant processare to be distinguished from copolymers in that the final polymericproduct is made up of blocks or segments, each of which is substantiallya homopolymer of one of the olefins employed in the process. The productof this invention can also be formed of blocks of copolymers andhomopolymers as well as blocks of copolymers only. Broadly speaking, theprocess of this invention comprises initially contacting at least onealiphatic 1-olefin with a catalyst comprising a hydride or organocompound of aluminum, gallium, indium or thallium and a di- or trihalideof a Group IV-A metal so as to form a polymer block, and, afterpolymerization of substantially all of the 1-olefin, contacting theafore-mentioned catalyst in the presence of the polymer block initiallyformed with at least one aliphatic 1-olefin so as to form a polymerblock adjacent the first-mentioned polymer block, the adjacent polymerblocks being non-identical as regards the monomer units comprising eachblock. It is to be understood that the instant invention is applicableto block copolymers formed of homopolymers of two different aliphatic1-olefins as well as a block copolymer in which the individual polymerblocks are homopolymers and copolymers or copolymers only. Whenpreparing block copolymers formed of homopolymer and copolymer blocks,one of the olefins used in preparing a copolymer block can be the sameas the olefin which is contacted with the catalyst in producing ahomopolymer block. In the case of block copolymers formed only ofcopolymer blocks, at least one of the olefins used in preparing acopolymer block is different from the olefins employed in forming anadjacent copolymer block. While the instant invention is, in general,applicable to block copolymers containing two or more individual polymerblocks, it is preferred that the products contain a total of at leastfour blocks.

The hydride or organo compounds used in the catalyst system of thisinvention correspond to the formula MR_(x) wherein M is one of themetals aluminum, gallium, indium, or thallium, R is hydrogen, amonovalent saturated acyclic hydrocarbon radical, a monovalent saturatedcyclic hydrocarbon radical, a monovalent aromatic hydrocarbon radical orany combination thereof, and wherein x is equal to the valence of themetal. Examples of these compounds include Al(C₂ H₅)₃, Al(CH₃)₃, HAl(C₂H₅)₂, H₂ AlCH₃, Ga(C₃ H₇)₃, In(CH₃)₃, AlH₃, Al(C₆ H₁₃)₃, Al(CH₂ (CH₂)₁₈CH₃)₃, Ga(C₆ H₅)₃, In(C₆ H₅)₃, Tl(C₂ H₅)₃ and the like. These compoundscan also be used in the form of their known and stable organiccomplexes, such as complexes with ethers, thioethers, amines, alkalimetal hydrides, alkali metal alkyls or alkali metal aryls. Examples ofsuch complex compounds which can be used in the catalyst system of thisinvention are LiAlH₄, NaAl(CH₃)₄, NaGa(C₆ H₅)₄, NaIn(C₂ H₅)₄, and thelike.

In admixture with one or more of the MR_(x) compounds described above,the catalyst system of this invention comprises at least one group IV-Ametal (Mendeleef's periodic system) di- or trihalide. The di- ortrihalide of any of the group IV-A metals including titanium, zirconium,thorium and hafnium can be used. The di- and trichlorides, di- andtribromides, di- and triiodides and di- and trifluorides of the groupIV-A metals can be used in the catalyst composition either individuallyor as mixtures. The di- and trichlorides of titanium are preferredbecause they have a high activity in the process of this invention.

Among the catalyst compositions falling within the scope of thisinvention which are preferred are the following: a mixture of titaniumtrichloride and triisobutylaluminum; a mixture of titanium trichlorideand triethylaluminum; a mixture of titanium dichloride andtriisobutylaluminum; a mixture of titanium dichloride andtriethylaluminum; and a mixture of zirconium trichloride andtriisobutylaluminum.

The amount of the catalyst composition which is to be used in thepreparation of the block polymers of this invention can vary over a widerange. Relatively small amounts of the catalyst have been found toproduce the desired activating effect. In general, the concentration oftotal catalyst in the reaction zone will be within the range of fromabout 0.01 to 10 weight percent of the diluent present in that zone. Themol ratio of the group IV-A metal dihalide or trihalide to the MR_(x)compound is within the range of 0.01:1 to 3:1, preferably within therange of 0.05:1 to 0.5:1.

The polymerization process of this invention is carried out in thepresence of a diluent which is inert and liquid under the conditions ofthe process. Diluents which can be used in the process includeparaffins, cycloparaffins and/or aromatic hydrocarbons. Examples ofsuitable hydrocarbon diluents are hexane, heptane, isooctane,cyclohexane, methylcyclohexane, benzene, toluene, and the like.

The polymerization process for this invention is usually carried out ata temperature within the range of 50° to 350° F. However, it is to beunderstood that higher and lower temperatures can be employed withoutdeparting from the spirit and scope of the invention. It is preferred tocarry out the process at a temperature in the range of 200° to 250° F.Although pressures ranging from atmospheric up to 5,000 psig can beused, a pressure in the range of 100 to 1,000 psig is ordinarilypreferred. In general, the pressure used in the instant process issufficient to maintain the reaction mixture substantially in the liquidphase.

The materials which are polymerized in accordance with this inventionare aliphatic 1-olefins. It is preferred to utilize olefins containingnot more than 8 carbon atoms per molecule. Examples of such olefinsinclude ethylene, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene.These alpha-olefins can be represented by the formula:

    CH.sub.2 = CHR

wherein R is selected from the group consisting of hydrogen and linearalkyl radicals containing 1 to 6 carbon atoms.

The polymerization process of this invention is conducted by initiallycharging the catalyst components and diluent to a suitablepressure-tight reaction zone. A 1-olefin is then charged to the reactionzone until a desired pressure therein is reached. The polymerization isallowed to proceed until substantially all of this 1-olefin has beenpolymerized as indicated, for example, by the drop in pressure whichtakes place in the reaction zone. When joining block copolymers ofnormally gaseous olefins in the presence of a relatively high boilingsolvent, such as cyclohexane, it has been found that when the reactionzone pressure is below about 45 psig, preferably below about 40 psig,substantially all of the 1-olefins have been polymerized. At this time,the reaction zone is repressured with another 1-olefin, which isdifferent from the 1-olefin originally charged, and the polymerizationis again permitted to proceed until substantially all of this latter1-olefin has been polymerized. This procedure then can be repeated withalternating charges of different 1-olefins, or, if desired, any numberof different 1-olefins can be alternately introduced into thepolymerization zone after the preceding 1-olefin has been substantiallyall polymerized. It is preferred to carry out the instant process sothat at least 4 individual charges of monomers are used. It is necessaryto either use up substantially all of each charge of 1-olefin beforecharging another or to remove unpolymerized monomer before the additionof another monomer. Thus, it is to be understood that it is within thescope of the invention to allow the polymerization to proceed until aportion only of one of the 1-olefins has been used up after which theremainder of this 1-olefin is removed from the reaction zone. Another1-olefin is then added to the catalyst and polymer in the reaction zone.It is necessary to proceed in this manner in order to preventcopolymerization from taking place.

It is also within the purview of the invention to produce a blockcopolymer of a homopolymer of a 1-olefin with a copolymer of 1-olefins.When preparing such block copolymers, a mixture of 1-olefins isinitially introduced into the reaction zone containing catalyst anddiluent. After the copolymerization has been completed or unpolymerizedolefins have been removed from the reaction zone, another 1-olefin,which may be one of the 1-olefins contained in the mixture originallyintroduced into the reaction zone, is then charged to the reaction zone.The product which is thereby produced is a block copolymer of a 1-olefinwith a copolymer of 1-olefins. Polymers formed of blocks of copolymerscan also be obtained by utilizing a mixture of 1-olefins for eachindividual charge. In preparing the latter type of block copolymers, atleast one of the olefins in a charge is different from the olefins in asucceeding charge. An example of such a block copolymer is one in whichthe polymer chain is formed of alternating blocks of a copolymer ofethylene and propylene and a copolymer of ethylene and butene-1.

The block copolymers which are prepared in accordance with thisinvention can vary widely in their ratios of monomers. For example,block copolymers containing 10 percent by weight ethylene polymer and 90percent by weight propylene polymer can be prepared. Similarly, polymerscontaining 90 weight percent ethylene polymer and 10 weight percentpropylene polymer can be formed. As hereinbefore discussed, blockcopolymers of more than two polymerizable monomers can also be prepared.However, the copolymers which are preferred are those produced from two1-olefins. A block copolymer of especially good properties is obtainedby polymerizing ethylene and propylene by the process of this inventionso as to form a block copolymer containing 50 percent by weight ethylenepolymer and 50 percent by weight propylene polymer. The block copolymersof this invention possess excellent physical properties, particularly asregards their tensile strength and impact strength. On the other hand,it has been found that copolymers containing the same proportion ofmonomer units which have been prepared by the catalyst system of thisinvention have physical properties inferior to those of thecorresponding block polymers. The preparation of copolymers bypolymerizing a mixture of two olefins is to be distinguished from theinstant process wherein the polymerization of a single olefin iscompleted prior to the introduction of the second olefin into thereaction zone.

Various materials are known to inactivate the catalyst composition ofthis invention. These materials include carbon dioxide, oxygen andwater. It is highly desirable, therefore, that the monomers be freed ofthese materials, as well as other materials which tend to inactivate thecatalyst. Any of the known means for removing such contaminants can beused. Furthermore, it is preferred that the diluent employed in theprocess be substantially free of impurities such as water, oxygen andthe like. In this connection, it is desirable to remove air and moisturefrom the reaction vessel in which the polymerization is carried out.Although it is preferred to carry out the polymerization under anhydrousor substantially anhydrous conditions, it is to be understood that somewater can be tolerated within the reaction mixture. However, the amountof water which may be tolerated is insufficient to completely deactivatethe catalyst.

After the completion of the polymerization reaction, the total reactionmixture is then treated to inactivate the catalyst, as by washing withan alcohol. The alcohol-washing step is preferably carried out in acommunition zone, such as a Waring Blendor, so that a finely-dividedpolymer is thereby provided. The polymer is then separated from thealcohol and diluent by decantation or filtration after which the polymeris dried. The diluent and alcohol can be separated, for example, byfractional distillation, and reused in the process.

A more comprehensive understanding of the invention can be obtained byreferring to the following illustrative examples which are not intended,however, to be unduly limitative of the invention.

EXAMPLE I

In this run, a block copolymer of ethylene and propylene was prepared inaccordance with the process of this invention. Fifteen-hundredmilliliters of cyclohexane were charged to a 1-gallon stainless steelautoclave which was equipped with a stirrer and a jacket for circulatingheating or cooling fluid. Prior to introduction, the cyclohexane waspurified by hydrogenation followed by drying over silica-alumina andbubbling with prepurified nitrogen. Forty-five milliliters of a solutionof triisobutylaluminum in cyclohexane, which contained 0.19 grams to amilliliter, and 1.2 grams of titanium trichloride were charged to theautoclave.

The titanium trichloride was prepared by charging two grams of aluminumpowder and 44.4 grams of titanium tetrachloride to a glass tube,flushing the glass tube with nitrogen, sealing the tube, and raising itstemperature to between 400° and 435° F. for 18 hours. The resultingproduct was a purple powder which was then transferred to another glasstube and maintained at a temperature between 400° and 500° F. for 18hours under a vacuum. The product of this latter heating step was thetitanium trichloride which was used as one of the components in theabove-described catalyst system.

After charging of the catalyst components, the autoclave was flushedwith nitrogen and heated to 220° F. After the nitrogen had been bledoff, the autoclave was alternately charged with ethylene and propylene.The ethylene and propylene were charged from cylinders which weremounted on balances, both streams being passed through a dry tube ofactivated silica-alumina before passing into the autoclave. Because ofthe weighing procedure, the amounts of monomers charged to the autoclaveare only approximate amounts. After an initial ethylene charge of 60grams, the pressure within the autoclave was 150 psig. Ten minutes laterwhen the pressure had fallen to 60 psig, an additional 40 grams ofethylene were charged to the autoclave, by raising the pressure to 150psig. Forty minutes later, the pressure had fallen to 30 psig,indicating that substantially all of the ethylene had been polymerized.The pressure was then raised to 100 psig by pressuring 50 grams ofpropylene into the autoclave. During the next hour and 15 minutes, 50more grams of propylene were charged to the autoclave. After 2 morehours, the pressure had dropped to 35 psig, indicating thatsubstantially all of the propylene had been polymerized. At this time 50grams of ethylene were charged to the autoclave, raising the pressuretherein to 180 psig. During the next 40 minutes, 50 grams of ethylenewere charged, and at the end of this period the pressure had againdropped, indicating that substantially all of the ethylene had beenpolymerized. During the next 40 minutes, 100 additional grams ofpropylene were charged to the reactor. The temperature during thepolymerization was maintained between 200° and 230° F. After the lastcharge of propylene, the polymerization was allowed to proceed for 15minutes at which time the pressure in the autoclave was 90 psig. Thereaction was then terminated by circulating cooling water through thejacket of the autoclave. Thirteen hours later, the temperature in theautoclave was 90° F. and the pressure was 20 psig. A heating medium wasthen circulated through the autoclave jacket, and after 25 minutes thetemperature and pressure in the autoclave were 180° F. and 35 psig,respectively. The stirrer was then turned on. After 75 minutes, thestirrer was turned off, and cooling water was circulated through thejacket. After the unreacted olefin was vented, the autoclave was thenopened. The solid polymer which was present in the autoclave was removedand washed in a Waring Blendor with one liter of methyl alcohol. Thewashed polymer was then dried overnight in a vacuum oven at 60° C. About445 grams of dry polymer was obtained. This polymer was evaluated forphysical properties as shown in the table at the end of Example IV.

EXAMPLE II

In this run, ethylene and propylene were copolymerized in an autoclaveunder the same conditions as those set forth in Example I.Fifteen-hundred milliliters of purified cyclohexane, 45 milliliters ofthe triisobutylaluminum solution, and 1.2 grams of titanium trichloridewere charged to the autoclave. The materials and amounts of materialsused in this example are identical to those employed in Example I. Afterflushing the autoclave with nitrogen, the stirrer was turned on, and thereactor contents were heated to 220° F. by means of a heating mediumcirculated through the reactor jacket. After venting the nitrogen fromthe autoclave, ethylene and propylene were charged to the reactorthrough the same apparatus employed in Example I. Because of theapparatus used, there was a time interval between the charging of thepropylene and the charging of the ethylene. However, this interval wassmall, e.g., 2 or 3 minutes, so that a mixture of the monomers waspresent in the reactor at all times. The initial monomer charge was 25grams of propylene, which elevated the pressure to 55 psig. Two minuteslater, 25 grams of ethylene were charged, raising the pressure to 120psig. After 13 minutes, the pressure in the autoclave had dropped to 40pounds, and at this time 25 grams of propylene were charged to thereactor. Three minutes later, 25 grams of ethylene were added to theautoclave. In another 12 minutes, 25 grams of propylene were pressuredinto the autoclave, and after an additional 3 minutes 25 grams ofethylene were charged. Seventeen minutes later, 25 grams of propylenewere charged and 2 minutes later 25 grams of ethylene were charged.After another 18 minute interval, 25 grams of propylene were pressuredinto the autoclave, followed 2 minutes later by the charging of 25additional grams of ethylene. After 28 more minutes, 25 grams ofpropylene were charged and 2 minutes later, 25 more grams of ethylenewere added to the autoclave. Thirty-three minutes later, 25 grams ofpropylene were charged, and 3 minutes later, an additional 25 grams ofethylene were charged. The reaction was allowed to continue for 57minutes, at which time 25 grams of propylene were charged. This wasfollowed 3 minutes later by the charging of 25 grams of ethylene.Sixty-two minutes later, the reaction was stopped by circulating acooling medium through the jacket of the autoclave. During the entirecourse of the reaction, the temperature in the autoclave was maintainedat about 220° F. The maximum pressure reached during the run was 215psig while the total reaction time was 90 minutes. At the end of thereaction, excess monomer was vented from the autoclave which was thenopened. The solid polymer present in the autoclave was washed in aWaring Blendor with one liter of methyl alcohol after which it was driedfor 60 hours in a vacuum oven at 70° C. The yield of dry polymer wasabout 414 grams. The physical properties of this polymer were determinedas shown in the table at the end of Example IV.

EXAMPLE III

In this run, a block copolymer of polypropylene with a copolymer ofethylene and propylene was prepared. Fifteen-hundred milliliters ofpurified cyclohexane were charged to the autoclave of Example I.Forty-five milliliters of a solution of triisobutylaluminum andcyclohexane, which contained 0.29 grams per milliliter, and 1.2 grams oftitanium trichloride, prepared as described in Example I, were thencharged to the autoclave. After flushing with prepurified nitrogen, theautoclave contents were heated to 230° F. by circulating a heatingmedium through the jacket. The nitrogen was then bled off, and themonomers were charged as described hereinafter. The initial charge was50 grams of propylene, which raised the autoclave pressure to 90 psig.After 30 minutes, 25 grams of propylene were charged, followed twominutes later by a charge of 25 grams of ethylene. After the pressurehad dropped to 40 pounds, which was 43 minutes later, 50 grams ofpropylene were charged to the autoclave, raising the pressure to 120psig. Forty minutes later, the pressure had dropped to 40 pounds, and 25grams of propylene were then introduced, followed two minutes later by a25-gram charge of ethylene. This charging procedure raised the pressureto 150 psig. After an additional 23 minutes, the pressure had fallen to35 pounds, and 50 grams of propylene were then charged, raising thepressure to 115 psig. Eighty-five minutes later, the pressure had fallento 40 pounds, and 25 grams of propylene were charged, followed 2 minuteslater by a charge of 25 grams of ethylene. This raised the pressure inthe autoclave to 200 psig. After an additional 78 minutes, the pressurehad fallen to 45 pounds at which time 50 grams of propylene werecharged. After 1 hour, the pressure had dropped to 50 pounds from avalue of 115 psig, and 25 grams of propylene were then charged, followed2 minutes later by a 25-gram charge of ethylene. Twenty-three minuteslater the pressure had dropped to 50 pounds at which time the reactionwas stopped by circulating cooling water through the jacket. Theautoclave was allowed to cool overnight after which it was opened andthe polymer was removed. The polymer was washed in a Waring Blendor with1 liter of methyl alcohol, after which it was dried overnight in avacuum oven at 65° C. Approximately 470 grams of dry polymer wasobtained. The properties of this polymer are set forth hereinbelow inthe table following Example IV.

EXAMPLE IV

In this run, it was attempted to prepare a block copolymer of ethyleneand propylene employing a catalyst system consisting of titaniumtetrachloride and triisobutylaluminum. Fifteen-hundred milliliters ofprepurified cyclohexane were charged to the autoclave of Example I,followed by charges of 1.15 milliliters (1.99 grams) of titaniumtetrachloride at 3.45 grams of triisobutylaluminum. The autoclave wasthen closed and flushed with prepurified nitrogen. The autoclavetemperature was then raised to 170° F. by circulating a heating mediumthrough the jacket. Fifty grams of propylene were then charged to thereactor, raising the pressure to 70 psig. One hour later, the pressurein the reactor was reduced to 10 psig by venting unreacted propylene.Two minutes later, the pressure was raised to 150 psig by charging 50grams of ethylene. After 90 minutes, the pressure had fallen to 10pounds, and 50 grams of propylene were charged raising the pressure to90 psig. Fifty minutes later, the pressure had fallen to 15 pounds, and50 grams of ethylene were then charged, raising the pressure to 175psig. One hundred-fifty minutes later, a 50 gram charge of propylene wasused to raise the pressure from 10 to 100 psig. Eighty minutes later, a50 gram charge of ethylene was employed to raise the pressure from 20 to250 psig. Thirty-five minutes later, the pressure had fallen to 10 psig,and the reaction was stopped by circulating cooling water through thejacket. During the course of the reaction, the temperature wasmaintained between 150° and 170° F. while the maximum pressure duringthe run was 250 psig. Upon completion of this polymerization, thereactor was allowed to cool and then opened. About 1 liter of isopropylalcohol was then added to the reactor. The reactor contents were allowedto stand overnight after which the polymer was removed and washed twicein a Waring Blendor, using one liter of isopropyl alcohol in each wash.The polymer was then dried overnight under vacuum at 65° C., giving ayield of polymer of about 317 grams. The properties of this polymer areset forth hereinbelow in the table.

                                      TABLE                                       __________________________________________________________________________                                 PHYSICAL PROPERTIES OF POLYMERS FROM                                          Example I    Example II                          Ash, Wt. %                   0.38         0.61                                Inherent Viscosity.sup.1     3.441        1.956                               Melt Index.sup.2             0.446        3.066                               Density, gm/cc at room temperature                                                                         0.916        0.875                               Crystalline Freeze Point, ° F..sup.3                                                                245          *                                   Impact Strength, Foot Pounds/Inch Notch.sup.4                                                              3.58         *                                   Stiffness, psi               84,000       *                                   Hardness, Shore D             64           24                                 Tensile Strength, At Yield, Compression Molded, psi.sup.5                                                  2048         *                                   Tensile Strength, At Yield, Injection Molded, psi.sup.6                                                    3170         *                                   Tensile Strength, At Break, Compression Molded, psi.sup.5                                                  Not Determined                                                                             194                                 Tensile Strength, At Break, Injection Molded, psi.sup.6                                                    Not Determined                                                                             *                                   Elongation, %, At Break, Compression Molded.sup.5                                                          5            820                                 Elongation, %, At Break, Injection Molded.sup.6                                                             26          *                                   Heat Distortion Temperature, ° F..sup.7                                                             112          *                                   Flex Temperature, ° F..sup.8                                                                        +21          *                                   Zero Strength Temperature, ° F..sup.9                                                               308           80                                                              PHYSICAL PROPERTIES OF POLYMERS FROM                                          Example III  Example IV                          Ash, Wt. %                   0.39         0.054                               Inherent Viscosity.sup.1     1.848        Not Determined                      Melt Index.sup.2             2.212        4.994                               Density, gm/cc at room temperature                                                                         0.891        0.877                               Crystalline Freeze Point, ° F..sup.3                                                                No Plateau   Not Determined                      Impact Strength, Foot Pounds/Inch Notch.sup.4                                                              8.37         *                                   Stiffness, psi               25,000       *                                   Hardness, Shore D             44           17                                 Tensile Strength, At Yield, Compression Molded, psi.sup.5                                                  1,297        Not Determined                      Tensile Strength, At Yield, Injection Molded, psi.sup.6                                                    No Yield     Not Determined                      Tensile Strength, At Break, Compression Molded, psi.sup.5                                                  1,297        207                                 Tensile Strength, At Break, Injection Molded, psi.sup.6                                                    1,900        226                                 Elongation, %, At Break, Compression Molded.sup.5                                                           30          188                                 Elongation, %, At Break, Injection Molded.sup.6                                                            120          135                                 Heat Distortion Temperature, ° F.sup.7                                                               95          *                                   Flex Temperature, ° F..sup.8                                                                        -28          -62                                 Zero Strength Temperature, ° F..sup.9                                                               273          150                                 __________________________________________________________________________     *Polymer sample too soft for test.                                            .sup.1 Inherent Viscosity measured on a solution of 0.2 gram of polymer i     50 cc of tetralin at 130° C.                                           .sup.2 ASTM D1238-52T                                                         .sup.3 Obtained by melting polymer sample, inserting thermocouple in          molten polymer, and allowing to cool slowly. Temperature is plotted on a      temperature v. time chart, and crystalline freeze point is the first          plateau in this curve.                                                        .sup.4 ASTM D256-47T                                                          .sup.5 ASTM D412-51T                                                          .sup.6 ASTM D638-52T                                                          .sup.7 ASTM D648-45T                                                          .sup.8 Determined by method of Clash and Berg, Industrial & Engineering       Chemistry, 34, 1218 (1942); recorded as temperature at which modulus in       torsion is 135,000 psi.                                                       .sup.9 Determined by method of Islyn Thomas, "Injection Molding of            Plastics, P. 504, Reinhold Publishing Co. (1947).                        

Referring to the above table, the polymers of Examples I and III wereprepared in accordance with the process of this invention while thepolymers of Examples II and IV were prepared by other processes and canbe considered as control runs. The polymer of Example II was acopolymer, and the data indicate that this polymer was inferior to theblock copolymer of Example I. The polymer of Example III is a blockcopolymer of polypropylene with an ethylene-propylene copolymer. ExampleIV illustrates that a catalyst system employing titanium tetrachlorideas one of the catalyst components is not effective in producing apolymer having the superior physical properties of the block polymers ofthis invention. Thus, the data in the above table show that in ExampleIV a polymer was produced which was too soft to determine its stiffness,hardness, or heat distortion temperature and which had a low tensilestrength and zero strength temperature as compared to the products ofthis invention.

The block copolymers produced in accordance with this invention haveutility and applications where solid plastics are used. They can bemolded to form articles of any desired shape, such as bottles and othercontainers for liquids. Also, they can be formed into pipe or tubes byextrusion.

As will be evident to those skilled in the art, many variations andmodifications of the invention can be practiced upon consideration ofthe foregoing disclosure. Such modifications and variations are believedto be clearly within the spirit and scope of the instant invention.

I claim:
 1. A block copolymer comprising first and second adjacentpolymer blocks, each of which is formed from one alpha-olefin having theformula:

    CH.sub.2 = CHR

wherein R is selected from the group consisting of hydrogen and linearalkyl radicals containing 1 to 6 carbon atoms, said polymer blocks beingnon-identical with respect to the alpha-olefins forming said blocks. 2.A block copolymer according to claim 1 comprising a first polymer blockof a homopolymer of ethylene and adjacent thereto a second polymer blockof a homopolymer of propylene.
 3. A block copolymer comprising first andsecond adjacent polymer blocks, each of which is formed from at leasttwo alpha-olefins having the formula:

    CH.sub.2 = CHR

wherein R is selected from the group consisting of hydrogen and linearalkyl radicals containing 1 to 6 carbon atoms, said polymer blocks beingnon-identical with respect to the alpha-olefins forming said blocks. 4.A block copolymer according to claim 3 comprising a first polymer blockof a copolymer of ethylene and propylene and adjacent thereto a secondpolymer block of a copolymer of ethylene and butene-1.